Potassium tert-butoxide

Potassium tert-butoxide is the chemical compound with the formula K+(CH3)3CO−. This colourless solid is a strong base (pKa of conjugate acid around 17), which is useful in organic synthesis. It exists as a tetrameric cubane-type cluster. It is often seen written in chemical literature as potassium t-butoxide. The compound is often depicted as a salt, and it often behaves as such, but it is not ionized in solution.

Potassium tert-butoxide
Skeletal formula of potassium tert-butoxide
Ball-and-stick model of the cubane tetramer that potassium tert-butoxide adopts in
Names
IUPAC name
potassium 2-methylpropan-2-olate
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.011.583
UNII
Properties
C4H9KO
Molar mass 112.21 g mol−1
Appearance solid
Melting point 256 °C (493 °F; 529 K)
Solubility in diethyl ether 4.34 g/100 g (25-26 °C)[1]
Solubility in Hexane 0.27 g/100 g (25-26 °C)[1]
Solubility in Toluene 2.27 g/100 g (25-26 °C)[1]
Solubility in THF 25.00 g/100 g (25-26 °C)[1]
Hazards
Safety data sheet Oxford MSDS
Harmful (Xn), Corrosive (C)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Preparation

Potassium t-butoxide is commercially available as a solution and as a solid, but it is often generated in situ for laboratory use because samples are so sensitive and older samples are often of poor quality. It is prepared by the reaction of dry tert-butyl alcohol with potassium metal.[2] The solid is obtained by evaporating these solutions followed by heating the solid. The solid can be purified by sublimation at 220 °C and 1 mmHg. Sublimation can also take place at 140 °C and 0.01 hPa. It is advisable to cover the raw material with glass wool, as potassium tert-butanolate tends to "bounce", so parts can be thrown up during the sublimation. The anhydrous removal using an inert sublimation apparatus is particularly advantageous.

Structure

Potassium tert-butoxide crystallises from tetrahydrofuran/pentane at −20 °C as [tBuOK·tBuOH]∞, which consists of infinite one-dimensional chains linked by hydrogen bonding. Sublimation of [tBuOK·tBuOH]∞ affords the tetramer [tBuOK]4, which adopts a cubane-like structure. Mild Lewis basic solvents such as THF and diethyl ether do not break up the tetrameric structure, which persists in the solid, in solution and even in the gas phase.[3]

Applications

The tert-butoxide species is itself useful as a strong, non-nucleophilic base in organic chemistry.[4] It is not as strong as amide bases, e.g. lithium diisopropylamide, but stronger than potassium hydroxide. Its steric bulk inhibits the group from participating in nucleophilic addition, such as in a Williamson ether synthesis or an SN2 reaction. Substrates that are deprotonated by potassium t-butoxide include terminal acetylenes and active methylene compounds. It is useful in dehydrohalogenation reactions.

Potassium tert-butoxide catalyzes the reaction of hydrosilanes and heterocyclic compounds to give the silyl derivatives, with release of H2.[5]

Modifications

Many modifications have been reported that influence the reactivity of this reagent. The compound adopts a complex cluster structure (the adjacent picture is a simplified cartoon), and additives that modify the cluster affect the reactivity of the reagent. For example, DMF, DMSO, hexamethylphosphoramide (HMPA), and 18-crown-6 interact with the potassium center, enhancing the basicity of the butoxide. Schlosser's base, a mixture of the alkoxide and an alkyl lithium compound, is a related but stronger base.[4]

Reactions

Potassium tert-butoxide reacts with chloroform yielding dichlorocarbene,[6] the reaction can result in ignition.[7] Potassium tert-butoxide should never be added to dichloromethane.[8]

As a base, potassium tert-butoxide can extract a beta-proton and form the Hofmann product via an elimination reaction. This reaction has a high synthetic value as it can set up further reactions of the resultant alkene, especially regiochemical reactions.

References
  1. Caine D. (2006). "Potassiumtert-Butoxide". Potassium tert-Butoxide. e-EROS Encyclopedia of Reagents for Organic Synthesis. doi:10.1002/047084289X.rp198.pub2. ISBN 0471936235.
  2. William S. Johnson and William P. Schneider (1963). "β-Carbethoxy-γ,γ-diphenylvinylacetic acid". Organic Syntheses.; Collective Volume, 4, p. 132
  3. Chisholm, Malcolm H.; Drake, Simon R.; Naiini, Ahmad A.; Streib, William E. (1991). "Synthesis and X-ray crystal structures of the one-dimensional ribbon chains [MOBut·ButOH]∞ and the cubane species [MOBut]4 (M = K and Rb)". Polyhedron. 10 (3): 337–345. doi:10.1016/S0277-5387(00)80154-0.
  4. Drury Caine "Potassium t-Butoxide" in Encyclopedia of Reagents for Organic Synthesis John Wiley & Sons, New York, 2006. doi: 10.1002/047084289X.rp198.pub2. Article Online Posting Date: September 15, 2006
  5. Anton A. Toutov, Wen-Bo Liu, Kerry N. Betz, Alexey Fedorov, Brian Stoltz, Robert H. Grubbs (2015). "Silylation of C–H bonds in aromatic heterocycles by an Earth-abundant metal catalyst" (PDF). Nature. 518 (7537): 80–84. doi:10.1038/nature14126. PMID 25652999. S2CID 3117834.CS1 maint: multiple names: authors list (link)
  6. Brown, William; Foote, Christopher; Iverson, Brent; Anslyn, Eric (2008-01-10). Organic Chemistry. Cengage Learning. ISBN 978-0495388579.
  7. Margaret-Ann Armour (2016-04-19). Hazardous Laboratory Chemicals Disposal Guide, Third Edition. CRC Press. ISBN 9781420032383.
  8. Foden, Charles R.; Weddell, Jack L. (1991-12-29). Hazardous Materials: Emergency Action Data. CRC Press. ISBN 9780873715980.
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